Recently, the requirement for new renewable energy and the interest of the new renewable energy have been more increased due to the serious environmental pollution and the lack of fossil fuel. In this regard, a solar cell is spotlighted as a pollution-free energy source for solving the future energy problem because it rarely causes environmental pollution and has the semi-permanent life span and there exists infinite resources for the solar cell.
Solar cells may be defined as devices for converting light energy into electric energy by using a photovoltaic effect of generating electrons when light is incident onto a P-N junction diode. The solar cell may be classified into a silicon solar cell, a compound semiconductor solar cell mainly including a group I-III-VI compound or a group III-V compound, a dye-sensitized solar cell, and an organic solar cell according to materials constituting the junction diode.
A solar cell made from CIGS (CuInGaSe), which is one of group I-III-VI Chalcopyrite-based compound semiconductors, represents superior light absorption, higher photoelectric conversion efficiency with a thin thickness, and superior electro-optic stability, so the CIGS solar cell is spotlighted as a substitute for a conventional silicon solar cell.
A CIGS thin film solar cell is fabricated by sequentially forming a substrate including sodium (Na), a back electrode layer, a light absorbing layer, and a thin film of a window layer. As described above, the light absorbing layer includes CIGS compound. When the CIGS compound is formed on the back electrode layer, a molybdenum selenide (MoSe2) layer is formed between the back electrode layer and the light absorbing layer.
The MoSe2 layer increases adhesive strength at the interfacial surface between the back electrode layer and the light absorbing layer, but has resistance higher than that of the back electrode layer to increase contact resistance between the window layer and the back electrode layer so that overall efficiency of a solar cell is significantly reduced.